The Piedmont and the coast have a few too many pine and cedar trees to get the classic forest-on-fire look that the mountains or places in the Northeast get, but they do get a major color tree not found in the mountains: the willow oak.

Willow oak trees are members of the red oak family native to the much of the Southeast, and the leaves take on a bright yellow-orange color in the fall, and they help give Raleigh the nickname, “City of Oaks.”

Emily Meineke, a post-doctoral researcher at Harvard, measured the growth of 40 willow oaks in Raleigh, looking for how temperature and the number of pests influenced their growth and photosynthesis rates.

Earlier studies by Meineke and others at NC State University found that higher temperatures increase the number of pest insects, because the parasites that prey on those pests can’t function as well at higher temperatures. Meineke and her colleagues designed an experiment to test whether variations in temperature would affect the pests feeding on willow oaks and the growth of the willow oaks themselves.

The researchers picked two willow oaks from each of 20 different sites around Raleigh. At each site, they applied a pesticide to one tree and left the other one bare. They sampled each tree for pests, trunk and branch growth and photosynthesis rate every six months for two years.

The average temperatures of the 20 sites all fell within a range of 3.6 degrees Fahrenheit, but even the small temperature differences between sites had dramatic effects.

The number of Parthenolecanium scale insects, the most common tree pest found in the willow oaks, increased linearly with temperature, and the warmest site had seven times as many scale bugs as the coolest. Similarly, when the average temperature reached about 61.5 degrees Fahreheit, populations of spider mites—another common willow oak pest—skyrocketed up to 56 times the population of the cooler sites.

The oil pesticide treatment cut the scale insect population by half and the spider mite population by two-thirds, but on the whole insect populations were higher at the warmer sites.

As predicted, the tree trunks in warmer areas grew more slowly than those in the cooler areas, but that decrease in growth was not explained by the explosion in pest population. In fact, the researchers found similar changes in growth rate in both the trees they treated for pests and the trees they did not. The pests only seemed to harm the willow oak branches, and that was only during one of the two years.

That means that while the warmer temperatures did lead to more insect pests, the high temperature separately slowed down the willow oak growth and photosynthesis rates.

Meineke and her colleagues found that the warmer areas tended to have less water available to the willow oaks, so there is some evidence that warming is related to a lack of water, but more studies will be needed to confirm that effect.

The fact that a slightly warmer urban landscape makes willow oak trees grow a little more slowly may not seem like an Earth-shattering discovery, but tree growth and climate are tied closely into each other.

The hotter willow oaks photosynthesize more slowly, which means they absorb less carbon from the atmosphere. Using a mathematical model, Meineke and her colleagues calculated that in Raleigh 2014, warm willow oaks absorbed 27 tons less carbon than they should ideally, a 12 percent reduction.

Carbon sequestration by plant life is one of the leading ways that carbon dioxide, a major contributor to the greenhouse effect and global warming, gets removed from the atmosphere. If rising temperatures are causing urban trees to sequester less carbon dioxide, then it could cause a feedback loop where decreased sequestration speeds up warming.

Further, Raleigh, like cities around the world, is what climate scientists refer to as an urban heat island—the technical term for the fact that it is always a few degrees hotter in cities and other populated areas than in rural communities or the wilderness. If warming is affecting carbon sequestration in other urban areas, which Meineke says is likely, urban trees and forests could be major contributors to increased carbon in the atmosphere. And as people continue to expand communities and develop new ones, urban forests are becoming a larger and more important means of absorbing carbon dioxide from the atmosphere.

If Meineke and her colleagues are correct about urban trees just being too thirsty, keeping them watered might provide a significant boost to how much carbon they can sequester. Water is a limited resource, however, so there would need to be a balance between the amount of water we use and the amount of carbon the trees can pull in.

More research will be needed to determine where that balance lies or if simply watering the trees is a helpful solution. Either way, any little change to help remove carbon from the atmosphere could help keep the willow oak colors going strong for many autumns to come.

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina.